The present invention relates to a magnetoresistive effect (MR) multi-layered structure especially using giant magnetoresistive effect (GMR) such as spin valve effect, and a thin-film magnetic head with the MR multi-layered structure used for a HDD (Hard Disk Drive) unit.
Recently, thin-film magnetic heads with MR sensors based on spin valve effect of GMR characteristics are proposed (U.S. Pat. Nos. 5,206,590 and 5,422,571) in order to satisfy the requirement for ever increasing data storage densities in today""s magnetic storage systems like HDD units. The spin valve effect thin-film structure includes first and second thin-film layers of a ferromagnetic material separated by a thin-film layer of non-magnetic metallic material, and an adjacent layer of anti-ferromagnetic material is formed in physical contact with the second ferromagnetic layer to provide exchange bias magnetic field by exchange coupling at the interface of the layers. The magnetization direction in the second ferromagnetic layer is constrained or maintained by the exchange coupling, hereinafter the second layer is called xe2x80x9cpinned layerxe2x80x9d. On the other hand the magnetization direction of the first ferromagnetic layer is free to rotate in response to an externally applied magnetic field, hereinafter the first layer is called xe2x80x9cfree layerxe2x80x9d. The direction of the magnetization in the free layer changes between parallel and anti-parallel against the direction of the magnetization in the pinned layer, and hence the magneto-resistance greatly changes and giant magneto-resistance characteristics are obtained.
The output characteristic of the spin valve MR sensor depends upon the angular difference of magnetization between the free and pinned ferromagnetic layers. The direction of the magnetization of the free layer is free to rotate in accordance with an external magnetic field. That of the pinned layer is fixed to a specific direction (called as xe2x80x9cpinned directionxe2x80x9d) by the exchange coupling between this layer and adjacently formed anti-ferromagnetic layer.
In this kind of spin valve effect MR sensor, the direction of the magnetization of the pinned layer may change in some cases by various reasons. If the direction of the magnetization changes, the angular difference between the pinned and free layers changes too and therefore the output characteristic also changes. Consequently stabilizing the direction of the magnetization in the pinned layer is very important.
In order to stabilize the direction of the magnetization by the strong exchange coupling between the pinned and anti-ferromagnetic layers, a process of temperature-annealing (pin anneal process) is implemented under an external magnetic field with a specific direction. The pin annealing is done by elevating the temperature up to the Neel point and then cooling down to room temperature under the magnetic field in the direction to be pinned. By this pin anneal process, the exchange coupling is regulated at the interface of the pinned and anti-ferromagnetic layers toward the direction of the externally applied magnetic field.
However, the magnetoresistance characteristics may be changed under actual high temperature operation of a HDD unit, even if the pin anneal processing is properly implemented. This degradation is caused by the high temperature stress during operation of the HDD unit and by the magnetic field by a hard magnet layer used for giving a bias magnetic field to the free layer.
The detail of this degradation is as follows. The pinned direction of the magnetization in the pinned layer is different from that of the magnetic field (HHM) by the hard magnet. And hence the direction of the magnetization of pinned layer which is contacted with the anti-ferromagnetic layer is slightly rotated toward the direction of HHM (hereinafter this direction of the magnetization of the pinned layer is expressed as xcex8p). In the anti-ferromagnetic material layer, the Neel point temperature differs from location to location inside the layer from macroscopic point of view, and it is distributed in a certain range of temperature. Even if the temperature is less than the xe2x80x9cbulkxe2x80x9d Neel point (average Neel point), there could be small area whose micro Neel point temperature is low and where the exchange coupling with the pinned layer disappears. When such spin valve effect MR sensor is operated at a high temperature T, which is less than the blocking temperature at which the exchange couplings of all microscopic areas disappear, and then cooled down to usual room temperature, some microscopic area whose Neel temperatures are less than T is effectively annealed again and the direction of the magnetization is rotated in the direction of xcex8p. The total amount of the xcex8p and the rotated amount component will change the exchange coupling state between the anti-ferromagnetic layer and the adjacent ferromagnetic layer to determine the new pinned direction of the magnetization of the magnetic structure. The new pinned direction will vary depending upon the period of time kept at high temperature because the magnetic characteristics of the ferromagnetic layer is changing over with time under the high temperature.
As stated in the above paragraph, usage of such spin valve MR sensor at high temperature may cause a change of the pinned direction of the magnetization in the pinned layer, and the electrical output characteristics of the sensor are degraded in signal levels, and waveform symmetry.
In order to prevent the above-mentioned problems from occurring, it had been desired that material having a high blocking temperature and possible to provide smaller Neel temperature distribution is used for the anti-ferromagnetic layer.
Japanese Patent Unexamined Publication No. 6(1994)-314617 discloses usage of a specific material for the anti-ferromagnetic layer to obtain improved exchange coupling. Also, Japanese Patent Unexamined Publication No. 9(1997)-82524 discloses insertion of an intermediate layer for improving lattice matching between the anti-ferromagnetic and pinned layers in order to obtain strong exchange coupling in the interface.
As disclosed in these publications, usage of the specific material for the anti-ferromagnetic layer and usage of intermediate layer between the interface of the anti-ferromagnetic and pinned layers have been proposed to enhance the exchange coupling and to stabilize the pinned direction. However, no one has approached to control the magnetic characteristics of the pinned ferromagnetic layer itself to stabilize the pinned direction.
It is therefore an object of the present invention to resolve the aforementioned problems by using a new approach, and to provide a MR multi-layered structure and a thin-film magnetic head with the MR multi-layered structure, whereby pinned direction can be kept in stable at high temperature.
According to the present invention, a MR multi-layered structure or a thin-film magnetic head with the MR multi-layered structure includes a non-magnetic electrically conductive material layer, first and second ferromagnetic material layer separated by the non-magnetic electrically conductive material layer, and an anti-ferromagnetic material layer formed adjacent to and in physical contact with one surface of the second ferromagnetic material layer, the one surface being in opposite side of the non-magnetic electrically conductive material layer. The second ferromagnetic material layer includes a first layer of a ferromagnetic material containing Co, and a second layer of a ferromagnetic material with a smaller magnetic anisotropy than that of Co. The first layer is stacked on the non-magnetic electrically conductive material layer, and the second layer is stacked on the first layer.
In other words, according to the present invention, the second ferromagnetic material layer (pinned layer) has a first layer which is made of a ferromagnetic material containing Co and stacked on the non-magnetic electrically conductive material layer, and a second layer which is made of a ferromagnetic material with a smaller magnetic anisotropy factor than that of Co and stacked on a surface of the first layer that faces to the anti-ferromagnetic material layer. Thus, the total magnetic anisotropy factor of the pinned layer is reduced to realize smaller magnetic variations under high temperature atmosphere, and hence a spin valve effect MR sensor with more stable direction of the magnetization of the pinned layer under high temperature atmosphere is realized. By stabilizing the direction of the magnetization of the pinned layer, the degradations of signal level and waveform symmetry of output waveforms under high temperature atmosphere (for example at about 125xc2x0 C.) can be greatly reduced.
It is preferred that the second layer is made of a ferromagnetic material of Fe alloy. In one embodiment according to the present invention, the ferromagnetic material of Fe alloy may be selected one of ferromagnetic materials of CoFe, FeSi and NiFe.
It is also preferred that the second layer is made of a ferromagnetic material of Ni alloy. In one embodiment according to the present invention, the ferromagnetic material of Ni alloy may be selected one of ferromagnetic materials of FeNi, NiCo and NiCu.
It is preferred that the second layer is made of an amorphous magnetic material alloy.
Preferably, the second layer has a thickness of 0.5 nm or more.
It is also preferred that the first layer is made of a ferromagnetic material of Co or CoFe.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.